The presence of a sheath surrounding at least a fraction of the last turn of the spring (the end turn) serves to avoid the spring making direct contact with the cup. Consequently, the noise coming from the end turn rubbing against the cup is attenuated and the vibration which can be transmitted to the rest of the vehicle is damped. In addition, the sheath serves to protect the end turn of the spring, in particular against corrosion which can arise in the vicinity of the contact between the end turn and the cup due to water and/or debris accumulating.
Spring suspension systems of this type are known comprising a tubular sheath covering the entire section of at least a fraction of the end turn. Nevertheless, such sheaths encourage water to condense, which can then penetrate inside the tube of the sheath and generally gives rise to corrosion of the turn. In addition, such a sheath is lengthy to mount since it needs to be threaded on via one of its ends.
Other spring suspension systems of this type are known comprising a sheath which presents an opening that enables the sheath to be fitted easily over the turn, while simultaneously avoiding water condensing around the entire section of the turn. Nevertheless, water and/or debris can accumulate in the bottom of the sheath, in particular for a bottom cup where the opening faces towards the top of the suspension system, and that leads to the turn being corroded.
In order to encourage removal of water and debris, sheaths are known in which the opening faces towards the outside of the spring. However, the turn is then exposed to being struck by chippings and that can damage the coating that is intended to protect the turn. In addition, it is difficult to mount such springs and to keep them in position since the outwardly-directed opening is contrary to the curvature of the spring.